This invention relates to an improved apparatus and a method used for the calibration of Almen gages used in measuring the intensity of shot blasting.
Gages of the general type disclosed in U.S. Pat. No. 2,350,440 (Almen), commonly referred to as "Almen gages," have been used for many years to measure the intensity of shot blasting. Almen gages measure the curvature of thin metal test strips after one side of the strips have been exposed to shot particles. A measurement is effected by measuring the displacement of the center of the test strip with respect to a zero datum. Almen gages are precision instruments and must be extremely accurate. Accordingly, Almen gages must be properly calibrated when manufactured and must be recalibrated on a regular basis. Since the purpose of the Almen gage is to provide uniformity in the shot peening process, it is desirable that gages used to measure the process should have as little variation from gage to gage as possible. It is common in shot peening operations for a shop to have multiple Almen gages, which are used interchangeably. However, different gages may have different readings, due to tolerance stack
Almen gages consist of a platform and a digital or analog indicating device which is mounted on the platform. Present practice is to separately calibrate the indicating device and the platform. The platform includes a measurement surface which includes a semispherical elements extending from the measurement surface to support the Almen strip and four posts engaging edges of the Almen strip to properly locate the Almen strips in the measurement position. Since the accuracy of the Almen gage is a direct function of the placement of the semispherical projections and the posts, proper placement of these elements must be verified within strict tolerances, usually by highly sophisticated coordinate measuring devices. Similarly, the indicator must be calibrated to within tight tolerances. However, since the indicator and the platform may be anywhere within the allowed tolerances, significant differences in readings gage-to-gage may occur. As discussed above, particularly in shops using multiple gages, it is highly desirable that these variations be minimized.